<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* id选择器优先级高于class选择器 */
        #outer{
            color: green;
        }
        .p1{
            color: red;
        }
    </style>
</head>
<body>
    <!-- class属性值允许重复 -->
    <p class="p1" id="outer">
        class01
    </p>

    <div class="p1">
        class02
    </div>
</body>
</html>